The player controls the King of Albion, who seeks to expand his kingdom north and conquer the Celts. With the backing of the Pope, the three most powerful dukes, and a horde of laborers, carpenters and craftsmen, the player must battle the Celts and rule all of Bretagne!
The central gameplay mechanic is the designing, construction and defense of the player's castles. At the beginning of the game, the player chooses either a 1, 3, or 8 castle campaign. Each successive level is more challenging, due to more difficult and frequent attacks, and less-hospitable terrain. The player is provided with an overhead view of the terrain of which he is looking to conquer. He can then choose from several pieces to place, and design his/her castle. These pieces include, towers, walls, gates, and moats. Layout choices will directly affect how well the castle can defend against enemies.
After the player has designed his castle, he must go about the business of managing his kingdom via the menus that are available to him/her. The player can alter taxes, hire or dismiss labor, levy military troops and buy food to prepare for a siege. Castle walls must be built one level and one section at a time. Battlements can be designed with defensive elements such as arrow slits and boiling cauldrons of oil.
The time of year has an effect on gameplay, as castle construction must halt during the winter, taxes are collected every spring, laborers and military units are hired over time, etc... Other events also rely on the passage of time.
Every month, a messenger arrives and brings you news of something that has occurred within the player's kingdom. The player is then given several choices in how to respond to these occurrences, with each choice resulting in a meaningfully different outcome. These choices are presented via a pop-up screen, with an image of the king thinking to himself about the choice at hand.
The player will suffer frequent attacks by the Celts, and other adversaries, especially in later levels. When this occurs, the player must place his gathered armies using any number of strategies (hide in the castle with archers on the walls, a large force of ground units at a choke point in the castle, small groups of units working together, etc), and then allow for the combat to play out. Unfinished castle sections are much less effective at keeping the enemy out. Enemy units will arrive from an unknown direction and begin trying to destroy the castle. If the player's castle is destroyed, the game is over.
Over time, the labor force of the Kingdom can be seen making progress on the player's castle. Progress is based on the happiness of the players population as well as the makeup of his labor force. Specialists are required to perform many of the building functions (wood workers for scaffolding and gates, stone masons for walls, etc). The player must decide how many resources to devote to workers/walls/moats/etc, resulting in many diverse and interesting gameplay possibilities that have a real impact on the outcome. When work the castle is completed, a larger battle will begin. If the castle survives, the player finishes that level, and moves on to a more difficult castle-building task at a new location.
The graphics in Castles are standard 256 color VGA. Individual laborers are given discernible feature (horses, tools etc...) along with accompanying animations. Building progress is shown in functional detail. Several more detailed and realized stills also exist within the game. Castle planning is done from a flat overhead view, while combat, and progress can also be watched from a 3/4th isometric perspective.
Castles features a fairly standard MIDI soundtrack.